MDEV-34825 FreeBSD fails to build under clang natively (postfix)

The fix in MDEV-34825/#3484/dff354e7df2f originally just took the FreeBSD carried
patch of inline ASM as it didn't have the __ppc_get_timebase function.

What clang does have is the __builtin_ppc_get_timebase, which was
replaced in the same commit, which was the fix taken from Alpine.

To reduce complexity - we only need one working function rather
than an equivalent asm implementation.

Noted by Marko, thanks!

Author: grooverdan

include/my_cpu.h

@@ -94,13 +94,8 @@ static inline void MY_RELAX_CPU(void)
   __asm__ __volatile__ ("pause");
 #endif
 #elif defined(_ARCH_PWR8)
-#ifdef __FreeBSD__
-  uint64_t __tb;
-  __asm__ volatile ("mfspr %0, 268" : "=r" (__tb));
-#else
-  /* Changed from __ppc_get_timebase for musl compatibility */
+  /* Changed from __ppc_get_timebase for musl and clang compatibility */
   __builtin_ppc_get_timebase();
-#endif
 #elif defined __GNUC__ && (defined __arm__ || defined __aarch64__)
   /* Mainly, prevent the compiler from optimizing away delay loops */
 #ifdef _aarch64_